1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those imaging functions, incorporating such a fixing device.
2. Description of the Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, an imaging process is followed by a fixing process using a fixing device, which permanently fixes a toner image in place on the recording medium by melting and settling the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image under heat and pressure.
One such fixing device includes a multi-roller fuser assembly that employs an endless fuser belt entrained around multiple rollers, paired with a pressure roller pressed against the outer surface of the fuser belt to form a fixing nip therebetween. The fuser belt is held on a heat roller internally equipped with a heater, which heats the length of the fuser belt through contact with the heat roller, so as to fix a toner image with heat from the fuser belt and pressure from the pressure roller through the fixing nip.
Another type of fixing device includes a film-based fuser assembly that employs a fuser belt formed of thin heat-resistant film cylindrically looped around a stationary, ceramic heater, which is paired with a pressure roller that rotates in pressure contact with the stationary heater through the fuser belt to form a fixing nip therebetween. The stationary heater heats the fixing nip, through which the pressure roller rotates to advance the fuser belt together with an incoming recording sheet, so as to fix a toner image in place with heat from the stationary heater through the fuser belt and pressure from the pressure roller.
The configuration based on the fuser belt combined with the stationary heater is commonly employed in a high-speed, on-demand printer, which can promptly execute a print job upon startup with significantly low energy consumption. Owing to the heat-resistant film which exhibits a relatively low heat capacity and therefore can be swiftly heated, this type of fixing device eliminates the need for keeping the heater in a sufficiently heated state when idle, resulting in shorter periods of wait time required to execute an initial print job upon startup, as well as smaller amounts of energy wasted during standby.
A problem common to the fuser assemblies described above is the difficulty in maintaining a fuser member at a consistent processing temperature along a circumferential direction in which the fuser member rotates in its generally cylindrical configuration. This is particularly true with the film-based assembly employing a fuser belt of low heat capacity locally heated with a stationary heater, which is vulnerable to periodic variations in temperature at the fixing nip, in particular, those caused by entry of a recording sheet absorbing heat from the fuser belt through the fixing nip, as well as fluctuations in temperature around a setpoint temperature, commonly called “ripples”. The problem is pronounced where the fuser belt is heated at idle without a recording sheet entering the fixing nip, which eventually causes various imaging failures, such as variations in gloss of a resulting image and undesirable transfer or offset of toner excessively heated at the fixing nip.
To cope with the problem, various methods have been proposed that control operation of a heater that heats a fuser member according to readings of a thermometer detecting temperature around the fuser member.
For example, one conventional method controls power supply of a heater according to temperature detected by a thermometer positioned upstream of the heater along the direction of rotation of a rotatable fuser member. According to this method, the thermometer is displaced with respect to the heater along the rotational direction by a distance determined based on a rotational speed at which the fuser member rotates and a response speed at which the thermometer responds to a change in temperature, so that the heater can properly heat each specific portion of the rotating fuser member with an appropriate amount of heat determined according to the output of the thermometer.
Another conventional method employs a pair of first and second thermometers around a cylindrically looped fuser belt, the former positioned at a center and the latter at an end of the fuser belt along an axial, longitudinal direction in which the fuser belt extends. According to this method, the first thermometer detects temperature of the fuser belt whereas the second thermometer detects temperature of the heater adjacent to the axial end of the fuser belt, so as to prevent the heater from overheating the fuser belt.
Although generally successful, neither of the conventional methods provides a satisfactory solution. That is, the former method fails to properly control the power supply during standby or upon startup, since, due to the absence of a thermometer positioned adjacent to the heater, it cannot detect temperature at the fixing nip where the fuser member is heated in a non-rotating, stationary state. On the other hand, the latter method has a drawback in that it cannot effectively maintain the fuser belt at a uniform temperature along the circumferential direction based on the output from the second thermometer detecting the operating temperature of the heater, where conducting heat from the heater to the fuser belt takes time to cause a substantial response delay.